Associations between children’s socioeconomic status and prefrontal cortical thickness Gwendolyn M. Lawson, Jeffrey T. Duda, Brian B. Avants, Jue Wu and Martha J. Farah

Introduction The goal of the present study is to investigate the relation between SES(socioeconomicstatus) and prefrontal cortical thickness in healthy children.They focus on prefrontal cortex for three reasons. First, this brain region is essential for executive function, which is associated with academic success. Second, the long developmental trajectory of prefrontal cortex and its sensitivity to environmental factors including stress suggest that differences in the experiences of lower and higher SES children could impact prefrontal development. Third, and most directly relevant, many studies have found SES differences in executive function and in prefrontal activity. They research if children who grow up in poverty tend to have lower Iqs and academic achievement scores and are less likely to develop basic reading and mathematics proficiency than their higher-SES counterparts.

Executive function is like the CEO of the brain. It’s in charge of making sure things get done from the planning stages of the job to the final deadline. When kids have issues with executive functioning, any task that requires planning, organization, memory, time management and flexible thinking becomes a challenge. the relevant published studies show that higher SES inchildren is accompanied by higher executive function and/or more mature or advantageous patterns of brain activity. Prior studies have quantified SES through parental education, total family income, family income-to- needs ratio and combinations of education and income measures. the current study used a measure of cortical thickness. Cortical thickness is defined in neuroimaging studies as the shortest distance between the white matter surface and pial gray matter surface.

Cortical thickness shown associations with cognitive ability and behaviour among healthy children. (Ducharme, Hudziak, Botteron, Albaugh,Nguyen, Karama, Evans & the Brain Development, Cooperative Group, 2012; Malek, Sharp, Greenstein, Rapoport &Giedd, 2011) To systematically investigate the relationship between socioeconomic status and cortical thickness in frontal brain regions, the current study used SES measures to predict cortical thickness in 10 prefrontal regions of interest (ROIs) in healthy children from the first time point of data collection in the NIH MRI Study of Normal Brain Development. The predictive power of family income and parental education were assessed separately because of recent literature suggesting that different measures of SES have unique relationships with cognitive outcomes and structural phenotypes.(Hanson et al., 2011; Noble et al., 2012).

Method This is a longitudinal study of typically developing children from ages newborn through young adulthood conducted by the Brain Development Cooperative Group and supported by the National Institute of Child Health and Human Development,the National Institute on Drug Abuse, the National Institute of Mental Health, and the National Institute of Neurological Disorders and Stroke. Participating children had been screened using rigorous demographic, prenatal history, physical, behavioral,family history, and neurological criteria.Data collection occurred at six pediatric study centers in major urban areas, and population-based sampling was used to obtain a demographically representative sample (Evans, 2006). A self-report measure of family income was obtained in 10 possible levels: from 0–$5,000 to $150,000.

Parental education level was measured in six possible categories for each parent: less than high school, high school, some college, college, some graduate level, graduate level. Out of 431 children with behavioral data, 283 children had available MRI data that met quality control standards as well as available data for all covariates used in analysis. Regions associated with executive functions found to differ as a function of childhood SES included left and right superior frontal gyri, left and right middle frontal gyri, left and right inferior frontal gyri,and left and right anterior cingulate gyri. Regions susceptible to stress also include the left and right anterior cingulate gyri and left and right orbitofrontal gyri.

Statistical approach Analyses used hierarchical linear regression executed in Statistical Package for the Social Sciences to predict cortical thickness in each region of interest from parental education and family income. Using an approach similar to Noble et al. (2012), potentially confounding variables were entered in the first step of a hierarchical linear regression model, and SES variables (parental education and family income) were added in the second step of the model. In the first step, ROI thickness was predicted from age (in days), sex, total brain volume, Full-Scale IQ (Wechsler,1999), body mass index (BMI), and child/race ethnicity (dummy-coded as ‘Non-White’ or ‘White’). While it has been argued that it is often unjustified to control for IQ in studies of neurocognitive outcomes (Dennis, Francis,Cirino, Schachar, Barnes & Fletcher, 2009)

Body mass index, calculated as (Weight in kg)/(height in m)2, was used as a covariate because it has been shown to significantly predict structural measures, including whole-brain gray matter and white matter volume in the visit 1 NIHPD data (Brain Development Cooperative Group, 2012). Children in the ‘Non-White’group (n = 72) had a mean family income of $63,841.16(SD = $34,481.63) and a mean parental education of 2.53(SD =.39) on the square-root transformed scale. Children in the ‘White’ group (n = 211) had a mean family income of $80, (SD = $31,503.54) and a mean parental education of 2.77 (SD =.42). Critically, in the next step, parental education and family income were added to the model.

Results The parental education and family income variables were significantly correlated with each other (r =.57,p <.001) and with full scale IQ (parental education r =.41, p <.001; family income r =.35, p <.001). Neither SES variable was significantly correlated with sex, age, BMI, or total brain volume (all p-values >.07). Without correcting for multiple comparisons, four prefrontal regions and one asymmetry measure showed significant relations with SES as measured by improved regression model fit when SES indices were added: Using a threshold of p <.05, a significant change in the model F statistic was found after adding parental education and family income to the model in the left anterior cingulate gyrus right anterior cingulate gyrus, left superior frontal gyrus, right superior frontal gyrus and superior frontal asymmetry measure.

To further investigate the differential ability of family income and parental education to predict cortical thickness, the model was repeated for the right anterior cingulate gyrus and left superior frontal gyrus using family income and parental education as independent predictors. In the right anterior cingulate gyrus, when controlling for age, sex, total brain volume, race, BMI, and IQ, parental education alone significantly predicted greater thickness while family income alone did not predict thickness. Using the same model to predict thickness in the left superior frontal gyrus, parental education alone significantly predicted greater thickness, while family income alone did not predict thickness.

DISCUSSION Within this large sample of healthy children, parental education predicted increased cortical thickness in the left superior frontal gyrus and right anterior cingulate gyrus, using a conservative threshold for statistical significance. While SES differences in behavioral measures of executive function and ERP measures of prefrontal cortical function have previously been documented, this study provides novel structural evidence for SES differences in selective regions of the prefrontal cortex. These findings add to the emerging literature suggesting that SES relates to structural brain variation, with other studies of healthy children reporting main effects of SES in the hippocampus and amygdala.

Longitudinal studies investigating the environmental and behavioral correlates of right ACC structure will be important to disentangle the relationship between environmental factors, brain development,and behavioral regulation ability. Noble et al.’s findings are not directly relevant as they concern gray matter volume, not thickness, in different prefrontal regions. Their data showed a trend toward a negative relation between parental education and volume at younger ages and a positive relation at older ages. One interesting and unexpected finding was the fact that parental education and family income, while highly correlated, showed strong differences in their ability to predict cortical thickness in frontal regions of interest. Parental education, but not family income, significantly predicted thickness in the right anterior cingulate gyrus and left superior frontal gyrus.

The difference between the predictive ability of parental education and family income provides support for the argument that SES indicators capture different aspects of environmental and genetic variation and should be treated separately but the mechanism for differences between parental education and family income is unclear. This difference may simply reflect differences in the sensitivity of the education and income scales in this dataset, or it may reflect meaningful differences in the genetic or environmental factors associated with these SES measures. One might expect that parental education would relate most closely to cognitive stimulation in the home environment (e.g. Hoff- Ginsberg & Tardif, 1995), while family income might be an important predictor of environmental stress exposure (e.g. Evans & English, 2002).

Thankyou for listening Alev Gündost